Liquid Crystalline Pressure-Sensitive Adhesive based on a Shape-Assisted Molecular Assembly with Electron-Deficient Twofold Columnar π-Stacking

Recently, the chemistry of supramolecular adhesives has rapidly progressed in materials science and tissue engineering. Many structural motifs with a variety of non-covalent interactions have been proposed for advanced adhesive properties. Here, we propose a new materials class of liquid crystalline pressure-sensitive adhesive (LC-PSA). Without the aid of hydrogen bonding or Coulomb force, a cyclooctatetraene(COT)-fused electron-deficient dipyridophenazine dimer (dppz-FLAP) forms a tight twofold columnar-stacking based on its V-shaped molecular structure (π-π distance: 3.32 Å). With the dppz-FLAP core as a mesogen of liquid crystal, a high shear LC-PSA bearing a well-defined packing structure in a hexagonal columnar phase has been developed. Both hydrophobic and hydrophilic glass substrates can be easily bonded at room temperature by simply pressing a flake sample of the molecular adhesive between the substrates. Tensile shear strengths reached approximately 1 MPa for glass, SUS and Fe substrates based on dispersion interaction with significant ductility, while the easy peelability on a PET tape was confirmed. The rigid columnar structure formed by the shape-assisted assembly results in the high cohesive force of the material, while the soft liquid crystalline properties provide sufficient fluidity as a PSA. Viscoelastic analysis revealed a unique position of the LC-PSA (G′~10^7 Pa, G′′~10^6 Pa) compared with conventional PSAs. The concept of LC-PSA based on the rigid/soft hybridization and hydrogen-bond-free molecular engineering extends the potential of supramolecular adhesives and functional small-molecule materials.